1. Field of the Invention
The present invention relates to a piezo-oscillator, and relates, more particularly, to a piezo-oscillator with an oscillation frequency of high stability.
2. Description of Related Art
A frequency of high stability has been required as a reference signal generation source in the fields of electrical machinery and apparatus such as frequency counters used for the maintenance of communication facilities in mobile communication base stations and satellite communication apparatuses, etc.
Accordingly, as a reference signal generation source, an oven controlled crystal oscillator is used having a frequency stability of about 1.times.10.sup.-7 to 1.times.10.sup.-10.
Further, in recent years, compact and light-weight portable apparatuses have been required in these fields, and accordingly, compact and light-weight properties have also been required in an oven controlled crystal oscillators.
An oven controlled crystal oscillator is comprised of a crystal resonator within a recess of a metal block having a large heat capacity for obtaining a highly stable oscillation frequency, and heating the metal block at a predetermined temperature of a constant level.
FIG. 5 is a partially cross-sectional perspective view for showing one example of a conventional oven controlled crystal oscillator.
As shown in the drawing, a recess 100a is formed in a metal block 100 made of a metal such as aluminum or the like, and a crystal resonator 101 is provided within the recess 100a.
A lead terminal of this crystal resonator 101 is connected to a substrate 102 on which an oscillation circuit is structured, by solder or the like.
A heating wire 103 for heating the metal block 100 is wound around the outer periphery of the metal block 100, and a semiconductor element 104 such as a transistor or the like is also closely adhered to the outer periphery of the metal block, for controlling power conduction of the heating wire 103.
With this arrangement, it is possible to utilize the semiconductor element 104 as an auxiliary heat source, and the characteristics of the semiconductor 104 can be stabilized by heating the semiconductor element 104 at a constant temperature by the heating wire 103.
Reference numeral 105 denotes a temperature sensor such as a thermistor or the like for detecting the temperature inside the metal block 100. Reference numeral 106 denotes a substrate on which a temperature control circuit is provided for controlling a heating temperature together with the semiconductor element 104 based on temperature information from the temperature sensor 105.
Reference numeral 107 denotes a metal vessel made of aluminum or the like, and a heat insulating material 108 is filled in the space within the metal vessel 107.
The oven controlled crystal oscillator having the above-described structure can make the heating wire 103 generate heat based on a control signal from the temperature control circuit 106 to heat the metal block 100 and the crystal resonator 101 at a specific constant temperature, so that it is possible to output a highly stable frequency signal.
However, the crystal resonator as shown in FIG. 5 has a problem that as it is necessary to use the metal block 100 having a predetermined heat capacity for holding the crystal resonator 101 at a constant temperature, manufacturing cost of the oscillator increases and the size of the oscillator becomes larger.
Further, as the substrate 102 has an extremely lower heat capacity as compared with the metal block 100, the substrate 102 is easily subjected to the influence of variations in the outer temperature. As the heat of the crystal resonator 101 is discharged to the lead terminal of the crystal resonator 101 and the substrate 102 connected to this lead terminal, the temperature of the crystal resonator varies according to the variation of the outer temperature, resulting in an unstable oscillation frequency.
The heating wire 103 starts heat generation after the power supply has been turned on, and the heat from the heating wire 103 is transmitted from the metal block 100 to a crystal blank via a long transmission route through a metal cap of the crystal resonator. Accordingly, it takes a long time until the temperature of the crystal resonator is stabilized. This causes a problem that it takes a long time from when the power supply voltage is turned on until when the oscillation frequency is stabilized, and that the power required for the heating becomes larger as the heat capacity of the metal block is large, so that low power consumption of the oven controlled crystal oscillator cannot be achieved.
The present invention has been made to solve the above-described problems, and it is an object of the present invention to provide a piezo resonator which can be manufactured in compact at low cost and which is excellent in starting characteristics, short starting time characteristics and low power consumption.